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The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, to the acquisition of data from the separate x-ray detectors in 2D detector arrays.
In a current computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the xe2x80x9cimaging plane.xe2x80x9d The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon a row, or one-dimensional array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a xe2x80x9cviewxe2x80x9d and a xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In one type of multi-slice CT imaging system 10 as shown in FIG. 1, the x-ray beam 14 also fans out along the z-axis producing a xe2x80x9ccone-beamxe2x80x9d. The detectors 22 are arranged in a two dimensional array 20 that has multiple rows of detectors to acquire attenuation measurements in a plurality of slices disposed along the z axis. The backprojected slice image is reconstructed from the volumetric data using the Feldkamp algorithm.
The reconstructed volume for an axial scan is a cylindrically shaped region. FIG. 2 illustrates a cross sectional view of the volume coverage at the iso-channel plane (a plane passing through both the x-ray focal spot 16 and the detector iso-channels and parallel to the z-axis). The desired reconstructed volume is represented by a dashed rectangle 50, having a height D that is equal to the height of the detector at the iso-center along the z-axis. From an image reconstruction point-of-view, every voxel, or element, in the image needs to be sampled by all projections to ensure artifact free reconstruction. However, the volume that satisfies this condition is depicted by a cross-hatched hexagon 52. In practice, the reconstruction volume for each scan is limited to the thick line rectangle 54 enclosed inside the cross-hatched hexagon 52 in order to obtain a continuous reconstruction volume with multiple axial scans. As a result, the distance between the adjacent axial scans cannot be larger than the dimension of this inner rectangle along the along the z-axis.
If the distance from the source to iso-center is denoted by S and the radius of the reconstruction field of view (x-y) is denoted by R, the distance t between adjacent axial scans for continuous coverage is given by the expression:                     t        ≤                              (                                          S                -                R                            S                        )                    ⁢          D                                    (        1        )            
Thus, to obtain continuous coverage of an organ within the patient being scanned, the distance between adjacent scans is limited to about half of the detector coverage in the z-axis at the iso-center. This significantly reduces the volume coverage capability of the scanner. Therefore it is desirable to increase the reconstruction volume to the full distance D. However doing so necessitates extensive extrapolation of the projection data and thus introduces significant image artifacts because the stippled triangular areas 56 and 58 at each corner of the desired reconstruction volume 50 are not fully scanned. As seen in FIG. 2, when the emitter 12 and detector array 20 are oriented as illustrated, the corner areas 58 closest to the detector array are within the x-ray beam 14. However, in this orientation, the triangular corner areas 56 closest to the emitter 12 are outside the x-ray beam 14. When the emitter and detector assembly has rotated 180xc2x0. i.e. have reversed the illustrated positions, corner areas 58 are outside the x-ray beam and corner areas 56 lie within the beam. As a consequence, the triangular corner areas 56 and 58 are only partially scanned during a complete rotation of the emitter and detector array. This requires extrapolation of the data for these areas which generates artifacts in the reconstructed image.
Therefore, although it is desirable to increase the size of the reconstruction volume as much as possible, ideally to distance D, doing so with conventional processes introduces significant artifacts into the reconstructed image. As a consequence, an alternative reconstruction process which reduces such artifacts is desired.
A computed tomography imaging system, includes a source of a conical beam of radiation and a multi-row detector array arranged on opposite sides of an axis of rotation. That imaging system employs an image reconstruction method which comprises rotating the source and detector about the axis of rotation. While that rotating occurs, x-ray attenuation data samples are collected from the multi-row detector array at a plurality of projection angles xcex2 thereby producing a set of projection data.
A half-scan image reconstruction technique is applied to the set of projection data at a plurality of different center-view angles xcex20 to produce a plurality of sub-images. The plurality of sub-images then are combined, such as by superimposition for example, to form a cross-sectional image of the object.
The preferred reconstruction technique includes weighting the set of projection data to produce a set of weighted data. For example, the weighting process applies a first weight to data samples within a first region centered about the respective center-view angle xcex20. A second weight is applied to data samples within predefined second regions on either side of the first region. A third weight is applied to data samples in the remainder of the set of projection data. The weighted data is filtered using a conventional filter for single slice scans. The filtered data are backprojected by known 3-D backprojection algorithms to produce a plurality of sub-images which are then combined by weighting functions to formulate the final image.